It's worth noting a great Android client for org-mode has been recently open sourced: http://www.orgzly.com/ https://github.com/orgzly https://f-droid.org/wiki/page/com.orgzly It was a little annoyance we did not have anything as good as this before.
I was wondering just a few days ago whether building a nice Mac desktop app or maybe a web app, akin to Ulysses, but for Org file writing/editing, exporting etc. would be a viable idea and whether it could eat some of Markdown's marketshare by not being tied to Emacs, which is why I feel most people don't pick it up. Would that be something the HN crowd would be interested in?
You can run Emacs headless, it has C extension support now, your Mac app could start it headless and communicate with it over a socket or as a network process. Multiple Emacs modes do this to make Emacs itself talk to other programs (e.g. mu4e mu, gud gdb), there's no reason you can't do it the other way around.